This invention deals with a method and structure for providing a signal which can rapidly switch among many frequencies.
Prior art attempts to provide a signal source that can rapidly switch among many frequencies have various disadvantages.
Multiple continuously running fixed frequency oscillators connected to an electronic switch for signal selection have been used when only a few output frequencies are required. This is impractical when many different output frequencies are needed. Also, the desired output frequencies must be known in advance--a severe disadvantage in applications such as electronic warfare (EW) jamming. When using multiple continuously running fixed frequency oscillators to generate output signals of accurate frequency, each oscillator must be separately synchronized to a reference frequency.
Slow switching signal sources have been multiplexed by means of fast electronic switches in order to select one signal source at a time. Off-line signal sources can change frequency and settle before they are selected by the switches. Unfortunately, multiplexed slow switching signal sources is an expensive and bulky solution. While this method can provide fast transitions from one frequency to another, it does not have the capability to switch often. Each signal source must dwell at a given frequency until its output signal has settled before its output signal becomes usable, thus limiting the frequency switching rate.
A phase lock loop indirect frequency synthesizer is capable of generating many frequencies, but has limited switching speed due to acquisiton, loop bandwidth, noise, and spurious signal considerations. Many phase lock loop frequency synthesizers require programable digital frequency dividers, which provide synthesizers which do not have phase memory. Phase memory is the ability to generate a signal, interrupt it, and at some later time again provide the signal having a phase indistinguishable from the phase the signal would have if the signal had been continuously present.
Mix-and-divide direct frequency synthesizers can be designed to switch frequency quickly. However, prior art methods of decoding frequency selection data to provide the necessary internal control signals are slow and rather complicated. Also, mix-and-divide direct frequency synthesizers do not have phase memory, since the signals driving the digital frequency divider circuits are interrupted during switching. The number of cycles of the interrupted signals which are missed due to switching is unknown. This undetermined time delay provides an uncertainty in the phase of the output signal.
Relevent prior art includes U.S. Pat. No. 3,838,355, "Frequency Synthesizers-A Survey of Techniques," Noordanus, IEEE Transactions on Communication Technology, Vol. Com-17, No. 2, April 1969, pp. 257-271; "A Submicrosecond Switching Frequency Synthesizer to 4 GHz," Papaieck, Microwave Journal, December 1975, pp. 133-136; "A 0-50 Mc Frequency Synthesizer with Excellent Stability Fast Switching, and Fine Resolution," Hewlett-Packard Journal, Vol. 15, No. 9, May 1964, pp. 1-6; "Digital Frequency Synthesizer Covering 0.1 MHz -500 MHz in 0.1 Hz Steps," Tykulsky, Hewlett-Packard Journal, Vol. 19, No. 2, October 1967, pp. 10-16: "Binary Frequency Synthesis: Signal Purity with Economic Simplicity," Oropeza, et al., Frequency, September-October 1966; "Multiple Synthesis Techniques Optimized Instrument's Performance," Feinberg, RF Design, May/June 1984, pp. 21-40; U.S. Pat. No. 4,272,730; "An Advanced Frequency Synthesizer Developed for Radar and Communications Applications," Rauvola, Microwave Systems News and Communications Technology, Vol. 17, No. 1, January 1987, pp. 8-12; an advertisement of John Fluke Manufacturing Company entitled, "The History of General Purpose Frequency Synthesizers," 1982; "Frequency Synthesizers-Direct, Indirect, or Direct Digital?," Hosking, Electronic Products Magazine, December 17, 1973, pp. 115-123; and "Why Complicate Frequency Synthesis?," Cooper, Electronic Design 15, July 19, 1974, pp. 80-84.